High-latitude obliquity forcing drives the Agulhas leakage

1. Influence of factors other than temperature on SST proxies: It is important to consider possible effects of fac tors other than temperature on the proxy records. For example, while TEX 86 (former form of H 86 TEX ) seems not to be affected by changes in salinity (Wuchter et al., 2004), it can be influenced by fluvial input of soil-derived isoprenoid GDGTs (Weijers et al., 2006). However, t he Branched and Isoprenoid Tetraether (BIT) Index (Hopmans et al., 2004) provides a metho d t assess the relative amount of soil organic matter input. In sediments with high BIT In dex values, it is likely that crenarchaeol and the other isoprenoid GDGTs are partially derive d from soils (Weijers et al., 2006). Thus, it has been suggested that TEX 86 only be applied in setting where the BIT Index is ~<0.3 (Weijers et al., 2006). In core MD96-2048, BIT valu es are extremely low (<0.1, Fig. S3D) indicating that the isoprenoid GDGTs have a predomi nantly marine source throughout the length of the record. Thus, an influence of fluvial input of soil-derived GDGTs on the TEX 86 values at our core site can be excluded. Some studi es have reported a seasonal bias on TEX 86 or K ́ 37 U records; however, this is likely not the case in th e Mozambique Channel region. Although modern sediments have not been studied exa ctly at the site of MD96-2048, a nearby sediment trap study (16.8°S, 40.8°E; 2250 m water d epth) (Fallet et al., 2010) provided some insights into application of these proxies in the M ozambique Channel, located upstream of site MD96-2048. At the Mozambique Channel sediment trap site, mean annual SST is 27.6°C, as measured by satellite remote sensing. The organi c proxies were found to reflect mean annual SST but seasonal variability was not reflect ed in either TEX86 or K ́ 37 U records or in fluxes of crenarchaeol or alkenones. In contrast, t he Mg/Ca records were found to track seasonal variability in SST and shell fluxes of G. ruber were found to be strongly seasonal, peaking in late austral summer (March).


Introduction
The AC is a key component of the global ocean "conveyor" circulation controlling the inter-ocean exchange of heat and salt (Agulhas leakage) (Weijer et al., 2002;Gordon, 2003;Lutjeharms, 2006;Beal et al., 2011).Modelling studies show that mesoscale eddies, so-called "Agulhas rings", transport and release warm and salty Indian Ocean waters into the South Atlantic, altering the buoyancy of Atlantic thermocline waters and Introduction

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Full influencing North Atlantic deep-water formation (Weijer et al., 2002) and, thus AMOC variability (Biastoch et al., 2008).Accurate knowledge of the mechanisms governing the dynamics and strength of the Agulhas leakage under different climatic conditions is therefore essential for properly constraining the long-term AMOC response to the Agulhas leakage in climate models (Gordon, 2003), for better understanding the evolution of global climate, and for credible long-term climate predictions.However, long-term SSS records are scarce in the Agulhas system and, to date, most of SST records were reconstructed either in the Agulhas ring region (Peeters et al., 2004;Martinez Mendez et al., 2010) or outside of the AC trajectory (Bard and Rickaby, 2009).
Here we present 800-kyr records of SST and changes in the δ 18 O of surface water (∆δ 18 O sw , a proxy of regional SSS) from a sediment core located beneath the present "precursor" region of the AC (MD96-2048, 26 • 10 482 S, 34 • 01 148 E, 660 m water depth, Fig. 1).We examine the effects of SST and SSS variations on changes in AC strength and Agulhas leakage.

Material and methods
Core MD96-2048 (37.59 m) was collected during the 104 MOZAPHARE oceanographic cruise of the R/V Marion Dufresne.This study was conducted on the top 12 m of core MD96-2048 which accumulated at an average rate of 2 cm kyr −1 .

Isotope analysis
The core was sampled every 2-5 cm for isotope analysis (δ 18 O).For each analysis, 4 to 6 specimens of planktonic G. ruber s. s. and benthic P. wuellerstorfi foraminifera were picked within the 250-315 µm size fraction.Analyses were carried out by a coupled system Multiprep-Optima of the mark Micromass at EPOC.The automated system of preparation (Multiprep) transforms carbonate samples (50 to 100 µg of calcium carbonate) to CO 2 gas evolved by treatment with ortho-phosphoric acid at a constant Introduction

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Full temperature of 75 • C. The CO 2 gas samples were then analysed by the isotope mass spectrometer (Optima) in comparison with a calibrated reference gas to determine the isotopic ratio 18 O/ 16 O of the sample.For all stable oxygen isotope measurements a working standard (Burgbrohl CO 2 gas) was used, which was calibrated against Vienna Pee Dee Belemnite (VPDB) by using the NBS 19 standard.Consequently, all δ 18 O data given here are relative to the VPDB standard.Analytical standard deviation is about 0.05 % (±1σ).

Mg/Ca analysis
Core MD96-2048 was sampled every 2-5 cm for Mg/Ca analysis.25 specimens of G. ruber s. s. were picked within the 250-315 µm size fraction for trace element analyses.Shells were cleaned to eliminate contamination from clays and organic matter based on the procedure of Barker et al. (2003).A Varian Vista Pro Inductively Coupled Plasma Optical Emission Spectrometer (ICP-OES) was used for magnesium and calcium analyses following the procedure established at LSCE (De Villiers et al., 2002).Reproducibility obtained from G. ruber s. s. samples was better than 4 % (±1σ, pooled RSD).For Mg/Ca ratios determined with a standard solution of Mg/Ca (5.23 mmol/mol), analytical precision was 0.5 % (±1σ, RSD).All the analyses were performed at LSCE, which participated in an inter-calibration exercise (Greaves et al., 2008).Measured Mg/Ca ratios were converted into temperature values applying the equation established by Anand et al. (2003) yielding a precision of 1.2 • C.

Alkenone and GDGTs analysis
Core MD96-2048 was sampled every 5-10 cm for alkenone and glycerol dialkyl glycerol tetraether (GDGT) analysis.Freeze-dried and grounded sediments were extracted with a Dionex Accelerated Solvent Extractor using column chromatography using hexane/DCM (9:1, v/v), hexane/DCM (1:1, v/v), and DCM/MeOH (1:1, v/v) as subsequent eluents.A known amount of an internal standard, a deuterated ante-iso C 22 alkane, was added to the alkenone fraction (hexane:DCM, 1:1 v/v) for quantification.The alkenone fraction (hexane/DCM, 1:1, v/v) was analyzed by a gas chromatography (Agilent 6890).The U K 37 was calculated as defined by Prahl and Wakeham (Prahl and Wakeham, 1987).The U K 37 values were converted into temperature values applying the culture calibration by Prahl et al. (1988) yielding a precision of 1.2 • C, which has also been validated by core-top calibration (M üller et al., 1998).The analytical precision of the method is about 0.3 The polar fraction (DCM/MeOH, 1:1, v/v), containing GDGTs, was analyzed using a high-performance liquid chromatography/atmospheric pressure chemical ionizationmass spectrometry.GDGTs were detected by single ion monitoring of their (M + H) ions and quantification of the GDGT compounds was achieved by integrating the peak areas and using the internal standard (C 46 GDGT).The TEX H 86 ratio was calculated as defined by Kim et al. (2010) and the TEX H 86 values were converted into temperature using the calibration of Kim et al. (2010) yielding a precision of 2.5 • C. The analytical precision of the method is about 0.2 • C.

δ 18 Osw reconstruction (proxy of SSS)
For the sea surface salinity (SSS) reconstruction, we followed the method developed by (Duplessy et al., 1991) which leans on the double influence of surface temperature and the δ 18 Osw isotopic composition of seawater on the isotopic values of the planktonic foraminifera (G.ruber s. s.).The isotopic temperature signals (Mg/Ca-SST) are subtracted from the planktonic δ 18 O record (G. ruber s. s.).The residual signal can be interpreted in terms of past δ 18 Osw variations (linked to SSS variations).An additional correction, linked to variation effect of continental ice (due to glacial-interglacial changes) has been applied to get the final ∆δ 18 Osw signal (Bintanja et al., 2005).Uncertainties of ∆δ 18 Osw estimates were obtained with an error propagation calculation Introduction

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Spectral estimates
For spectral estimates, the proxy records are linearly interpolated to a uniform spacing of 0.5 kyr.The power spectral density, coherence and phase are estimated using a smoothed periodogram (Bloomfield, 1976).Before the analysis, a split cosine bell taper is applied to 10 % of the data at the beginning and end of the series.To estimate the significance of the power spectral density, the spectrum background is estimated by fitting an analytical red noise spectrum to the median-smoothed spectrum estimate (Mann and Lees, 1996).The confidence intervals are calculated under the assumption that the spectral estimates are chi-square distributed (Percival and Walden, 1993).
The degrees of freedom of the spectral estimate are 8 for the power spectral density calculation and 19 for the phase and coherency calculation.Our statistical procedure of estimating the significance of the sample coherence and the confidence intervals of the phase estimate largely follows Huybers and Denton (Huybers and Denton, 2008).
To estimate the significance of the coherence, we use a Monte Carlo procedure to estimate the 95 % significance level.Therefore, one of the two time-series is replaced by a red-noise process using the estimated lag-1 auto-covariance.The coherence is estimated 10 000 times on the surrogate time series.The uncertainty in the phase estimated is also estimated using a Monte Carlo procedure.Using white noise realizations, a signal according to the degree of coherence estimated from the data is generated.
Here, the bias-corrected coherence estimate is used (Amos and Koopmans, 1963).
The algorithm is repeated 10 000 times to estimate the 95 % confidence intervals for the phase estimate.Introduction

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Results
Characteristic glacial-interglacial changes in the down-core record of stable oxygen isotopic composition of the benthic foraminifer Planulina wuellerstorfi (δ 18 O benthic , Fig. 2a) served as control points for the age model tuned to the LR04 stack (Lisiecki and Raymo, 2005) (Fig. S1), and allowed for the identification of eight terminations.The age model of Lisiecki and Raymo, (2005), constrains ages by aligning variations in the benthic foraminifera δ 18 O record with variations in the orbital parameters.It has been criticised that this approach precludes an objective evaluation of the orbital influence on glacial timing (Huybers, 2007).We therefore established an additional age model not relying upon orbital assumptions, by tuning the benthic δ 18 O record to the depth-derived age model (Huybers, 2007) (Fig. S1).In general, the H07-based age model strongly resembles the LR04-based one.However, both age models deviate from each other for the time interval between MIS 7 and MIS 9. We applied three independent inorganic and organic paleothermometers to reconstruct AC SST changes: Mg/Ca ratios of the surface-dwelling planktonic foraminifer Globigerinoides ruber sensu stricto, alkenone unsaturation index (U K 37 ) from haptophyte algae, and tetraether index (TEX H 86 ) of Group I Crenarchaeota.Measuring three independent proxies is important to crosscheck temperature variations.All three records are strongly related between each other (R > 0.5,p < 0.01) and exhibit typical glacialinterglacial patterns (Fig. S2).As each proxy has some uncertainty related to the calibration, non-temperature influences and lateral advection, the three records were averaged into a single SST stack (Fig. 2b).It is reasonable to assume that the uncertainties are independent between the proxy types.Therefore, the stack is a more accurate temperature reconstruction than the usual interpretation of single temperature proxy records.It also facilitates visual comparisons with other records and strengthens the common down-core patterns.Before stacking, U K 37 and TEX H 86 signals were linearly interpolated to the same time resolution as the Mg/Ca signal, which is the SST dataset with the highest time resolution.To examine whether the SST stack is representative Introduction

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Full of coherent down-core temporal variation, we applied Empirical Orthogonal Function (EOF) analysis (Von Storch and Zwiers., 1999) on the three SST records (Fig. S2).An almost identical temporal variation of the first Principal Component (PC1, 74 % variance) and the SST stack (R > 0.99) confirms that this record represents the common temporal variation of the three individual SST records.Additionally, we constructed past ∆δ 18 O sw (Fig. 2c), by combining the Mg/Ca SST estimates with the δ 18 O values of G. ruber and removing the influence of continental ice volume on global δ 18 O sw .

Orbital forcing for agulhas leakage records
Our SST stack and ∆δ 18 O sw records show glacial-interglacial patterns (Fig. 2b-c).
Spectral analyses revealed a strong signal in both 100-kyr (glacial-interglacial) and 41-kyr (obliquity) periodicities (Fig. 2d, 3a-b).The origin of the 100-kyr cycle could be linked to eccentricity forcing, to internally-driven climate feedbacks imparting some eccentricity influence (Lisiecki, 2010), or, alternatively, 100-kyr cycles can result from quantized bundles of 41-kyr obliquity cycles (Huybers and Wunsch, 2005).However, the 23-kyr and 19-kyr (precession) signals, which are modulated by eccentricity, are weak in our records (Fig. 3), suggesting that eccentricity forcing plays no significant role at our site.Interestingly, over the last 800 kyr, obliquity signals of SST stack and ∆δ 18 O sw records are nearly in phase with changes in high-latitude annual mean insolation rather than with any local insolation index at 26 • S (Berger and Loutre, 1991) (Fig. 2e) and with the obliquity components of the Antarctic temperatures (Jouzel et al., 2007) (Fig. 2f).Regardless of whether the statistical analysis is performed using an alternative age model (Fig. S4), or with the individual SST records (Fig. S5), the important finding remains that all records vary in phase with changes in high-latitude obliquity.This indicates that a strong influence of local insolation on our records can be excluded; a linear response to local insolation would be out of phase in the obliquity Introduction

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Full  , 2009), would contain a strong precession component contrary to our observation in the Agulhas records.An important role of the latitudinal insolation gradient (LIG) can also be excluded as it contains both obliquity and precession frequencies as a result of seasonal differences in orbital forcing (Davis and Brewer, 2009).In addition, the LIG has been suggested as the origin of obliquity periodicities evident in the deuterium excess record from the Vostok ice core in Antarctica (Vimeux et al., 1999).However, the deuterium excess record is out of phase in the obliquity band with the Agulhas records as with obliquity components of the Antarctic temperatures (Vimeux et al., 1999).
Changes in high latitude insolation driven by obliquity variations may have controlled the position of the STC along with the shift of the Southern Hemisphere westerlies (Bard and Rickaby, 2009;Biastoch et al., 2009), changes in heat export from the tropics (Jouzel et al., 2007), and sea ice coverage (Knorr and Lohmann, 2003).Poleward shifts of the STC modify recirculation in the Indian subtropical gyre (Bard and Rickaby, 2009), which intensifies heat and salt transfer from the Indian Ocean to the South Atlantic, thus increasing SSTs and SSSs of the AC.Recent observations (Alory et al., 2007) and modelling results (Biastoch et al., 2009) have shown warming/salinification tendencies of the southwest Indian Ocean in relation to a poleward migration of the STC.Interestingly, Antarctic temperature records (Jouzel et al., 2007) also exhibit strong obliquity components (Fig. 2f).Consequently, the in-phase relationship of the obliquity components of the Antarctic temperatures (Jouzel et al., 2009) and the SSTs and SSSs of the AC (Table 1) suggests that variability in AC is coupled to high latitude Southern Hemisphere climate forcing.It should also be noted that the Agulhas leakage fauna (ALF) record, a foraminiferal proxy of Agulhas leakage from the Cape basin (Peeters et al., 2004) and our ∆δ 18 O sw record do show, in addition to obliquity signals, weak signals in the precession band that is absent in our SST records (Fig. S6).Peeters and co-workers proposed a possible link of the Agulhas leakage with low-latitude monsoon forcing to explain the presence of precession signals.However, in contrast to Introduction

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Full the ALF record, the monsoon records are not in phase with the Northern Hemisphere summer insolation intensity (Clemens et al., 2010;Caley et al., 2011).This suggests that the increased strength of the Indo-Asian monsoon cannot fully explain the timing of the precession signal recorded in the ALF record.Interestingly, the STC record also contains a very weak precession signal (Peeters et al., 2004).This suggests that the precession signal in the Agulhas system might be linked to a high-latitude climate forcing via Southern Hemisphere frontal changes rather than by low latitude climate forcing originated from the Indian Ocean.

Subtropical convergence migration, agulhas current strength and transfer relationship
Recently, it has been suggested that northward migrations of the STC modulated the severity of each glacial period (particularly during MIS10 and 12 at site MD96-2077) (Bard and Rickaby, 2009).The hypothesis that a northward-migrating STC would block the AC and thus affect water transport from the Indian Ocean to the South Atlantic is still under debate (Rau et al., 2002;Bard and Rickaby, 2009;Zahn, 2009).For the particular and exceptional MIS 12 (extreme northward position of the STC) (Bard and Rickaby, 2009), all records at site MD96-2048 show an early increase in SST whereas the coldest SSTs are observed further to the south at site MD96-2077 (Figs. 1, 2).This suggests that the build-up of heat from the return flow of the AC is linked to the latitudinal contraction of subtropical gyres (Sijp and England, 2008).A comparison of our U K 37 SST record with that of MD96-2077 also reveals stronger deviations during the glacial periods, especially MIS 10 and 12 (Fig. 4a-b).Increased glacial SSTs were recorded at site MD96-2048 when the STC reached its northern most position, which may be related to a build-up of heat from the return flow that could not escape to the Atlantic as for MIS 12.Alternatively, lateral fluxes and thus the AC were stronger when Agulhas leakage was weaker (Supplement), which contradicts the existing hypothesis that reduced glacial Agulhas leakage was caused by a weakened AC (Franzese et al., 2009).Although our hypothesis awaits future confirmation, it is in good agreement with Introduction

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Full some modelling results showing that when the AC is weak, the Indian-Atlantic interocean exchange is larger with westward movement of the Agulhas retroflection (De Ruijter, 1982;Van Sebille et al., 2009).

Impact of heat and salt agulhas leakage
Our results show that changes in SST and SSS led variations in global ice volume (i.e.benthic δ 18 O value) on obliquity time-scales (Fig. 3c-d).The time-lags of benthic δ 18 O are 2.2 kyr (±1.9 kyr, 95 % confidence interval) for SST and 1.7 kyr (±6.1 kyr, 95 % confidence interval) for ∆δ 18 O sw in the obliquity band (Table 1).The ALF variations also led benthic δ 18 O changes in the Cape basin (Peeters et al., 2004).This suggests that the enhanced leakage of warmer and saltier Indian Ocean waters into the South Atlantic during the terminations allowed for the development of a South-North density gradient in the Atlantic before the global ice volume change, reinforcing the AMOC (Weijer et al., 2002;Biastoch et al., 2008).Recently, Lisiecki et al. (2008) showed that maxima in high northern latitude summer insolation (that is, Milankovitch forcing) are associated with greater mid-depth Atlantic overturning in the obliquity band but with less overturning in the precession band.This is in contrast to the SPECMAP hypothesis that circulation response has the same phase relative to ice volume in all three orbital bands and suggests that the AMOC is more strongly influenced by other factors than ice volume changes and summer insolation at high northern latitudes (Lisiecki et al., 2008).Interestingly, it has also been shown that stronger AMOC during MIS 11 inhibited significant ice-sheet build-up and prolonged the interglacial period at a time of high orbital obliquity (Dickson et al., 2009(Dickson et al., , 2010)).Our results from the Agulhas system provide an important metric for the AMOC response to orbital-obliquity forcing that contributed to global climate changes as a positive feedback.Obliquity-driven glacial terminations during the late Pleistocene have previously Introduction

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Full that amplified the initial obliquity forcing have not been elucidated yet.AMOC responses to orbital forcing are also highly model-dependent, showing contradictory results (Yoshimori et al., 2001;Khodri et al., 2003).Our finding of obliquity-driven Agulhas leakages sheds light on a new feedback mechanism for long-term AMOC responses to the inter-ocean heat and salt exchange.We suggest that this obliquity signal is transmitted from the Southern Hemisphere to the Northern Hemisphere via AMOC changes.This could explain why AMOC variability is not solely dependent on ice volume and summer insolation at high northern latitudes (Lisiecki et al., 2008).

Conclusion
Sea surface temperature and salinity records from the "precursor" region of the Agulhas current contain strong obliquity-driven 41-kyr cycles over the last 800 kyr.This suggests that long-term Agulhas leakage dynamics are associated with high latitude rather than a tropical climate forcing mechanism by varying the position of the Southern Hemisphere subtropical convergence and its associated westerlies.
To trigger ice age terminations, important feedbacks need to be added to the direct effect of insolation changes on ice sheets.We argue that the important transfer of heat and salt via the AC, which affected the resumption of the AMOC and the initiation of interglacial conditions (Weijer et al., 2002;Knorr and Lohmann, 2003;Biastock et al., 2008), is one of the main feedbacks.Intermediate complexity climate models emphasize the important role of Agulhas leakage for AMOC changes (Marsh et al., 2007), while fully coupled ocean-atmosphere models do not resolve the Agulhas leakage (Lohmann, 2003;Beal et al., 2011).Therefore, obliquity-induced variability of the Agulhas leakage merits greater attention in global ocean and climate models used for predicting the future climate scenarios.Figures

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Full Table 1.Phase and coherence between Agulhas surface proxies and −1 × δ 18 Obenthic.The asterisks indicate that the coherence is not significant (p = 0.05).A negative phase indicates that the Agulhas surface records are leading δ 18 Obenthic.In addition, phase and coherence between Antarctic temperature records and Agulhas SST stack record were calculated.A negative phase indicates that Agulhas SST stack is lagging Antarctic ice core records.Note that phase and coherence between the Vostok temperature record (Suwa and Bender, 2008) and Agulhas SST stack were calculated for an overlapping period (i.e.0-411 kyr BP).

Proxy
Frequency Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | a 9:1 (v/v) mixture of dichloromethane and methanol at NIOZ.After extraction, a known amount (1 µg) of C 46 GDGT internal standard was added to the total extracts.The extracts were separated by Al 2 O 3 Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | band with the Agulhas records and local seasonal responses, e.g.caused by local nonlinearities (Leapple and Lohmann Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | 2002.Yoshimori, M., Weaver, A. J., Marshall, S. J., Clarke, G. K. C.: Glacial termination: Sensitivity to orbital and CO 2 forcing in a coupled climate system model, Clim.Dyn., 17, 571-588, 2001.Zahn, R.: Beyond the CO 2 connection, Nature, 460, 335-336, 2009Discussion Paper | Discussion Paper | Discussion Paper |